When electroneutral or charged lipid bilayers approach each other to distances of ~1.5-0.5 nm one observes a strong repulsive force acting between them. This force is called solvation (hydration) since, as it was suggested it is due to the solvent (usually water). The strength of the repulsive force as well as its measured decay length strongly depend on the type of lipids and their physical state (Masurments by Rand and Parsegian and by McIntosh and Simon). Measurements by McIntosh and Simon of forces of interaction between membranes have been performed on systems in aqueous solvents, but it is also known that lipid bilayers swell in other solvents and the results obtained compare quite well with the aqueous case. In addition Leikin et al have shown that hydration forces can be observed to act between other surfaces, e.g. DNA polyelectrolytes and polysaccharides. All these facts make the interpretation of the nature of hydration force very complicated. Some researches ascribe the force to the ordering of solvent (water) by the surfaces of biomolecules. Such a mechanism is described by order parameter theories started by the work of Marcelja and Radic and extended by Kornyshev and Leikin, and Cevc. A completely different explanation for the origin of hydration force was given by Israelachvili and Wennerstrom, who proposed that the short range repulsion force is due to steric interactions of biomolecules that protrude into the fluid space. In this subproject we have performed a series of computer simulations designed to gain a better understanding of the nature of the hydration force and to distinguish between the two different mechanisms that have been proposed in explanation of this force. The important feature that appeared from our simulations is that the membrane/water interface is very rough (Essmann, et al., Langmuir 1995 and 1996). The width of the interfacial region measures about 1.0 nm, which means that the thickness of the two interfacial regions comprises 40% of the total membrane! Just based on these numbers one could already conclude that the interface has to be modeled in three dimensions. The roughness and dynamical nature of the interface smears out the oscillatory behavior that is expected from the molecular nature of the system. The decay of all properties concerning water ordering is therefore rather smooth. The largest decay occurs within the interface itself. Moreover, it seems to be determined by the decay of the interface. Since experiments by Wiener and White and our simulations show that membrane surfaces are rough on a molecular scale, steric interactions must play a part in repulsive interactions even when the distances between membranes are ~1 nm. That means that the mechanism proposed by Israelachvili and Wennerstrom must play a substantial role in the total repulsion force acting between membrane surfaces. From our simulations we also understand what causes the difference in the hydration force when the headgroups of the membranes are different. Thus we observed that even at the hydration limit of membranes with the PC (phosphatidylcholine) headgroups there are not more than two layers of water solvent (on the average) solvating each lipid molecule. For membranes with PE (phosphatidylethanolamine) molecules only one water layer on the average is involved. That means that all the water of hydration is disturbed by the membrane surface. Considering the difference in the structure of the head group of the membrane molecules (PE vs PC), we expect that the water structure in the space between membranes will also be different. And indeed we find this to be the case. (Around PE headgroups we observed that there is a tendency for water to create clathrate structures, while opposite surfaces of PE membranes are bridged through waters making hydrogen bonds connecting phosphate groups from one bilayer with NH3 groups from opposing bilayer.) The difference in water structure is used by us to explain why the hydration of PE and PC membranes is different. That means that detailed structure of water plays an important role in determining the hydration limit and explains why the hydration force is not universal in its character, contrary to the initial belief. At the same time, we conclude that special properties of water (ability to create hydrogen bonding structures) contribute to the hydration force. This is in agreement with the initial sugestion by Marcelja Radic. for the Resource Advisory Committee by Joe Quinn, Chair)